WM - Analytical techniques

Question 1

What does infrared spectroscopy help you identify?

Answer 1

Organic molecules

Question 2

What happens in infrared spectroscopy?

Answer 2

A beam of IR radiation is passed through a sample of chemical.

It is absorbed by the covalent bonds in the molecules, increasing their vibrational energy (i.e. they vibrate more).

Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different places in a molecule absorb different frequencies too.

Question 3

What (in IR spec) absorbs different wavelengths of IR radiation?

Answer 3

Bonds between different atoms absorb different frequencies of IR radiation. Bonds in different places in a molecule absorb different frequencies too.

Question 4

What absorbs the IR radiation in IR spec?

Answer 4

It is absorbed by the covalent bonds in the molecules, increasing their vibrational energy (i.e. they vibrate more).

Question 5

What happens when the covalent bonds in the molecules absorb the IR radiation in IR spec?

Answer 5

Their vibrational energy is increased (i.e. they vibrate more).

Question 6

What does an infrared spectrometer produce?

Answer 6

A spectrum that shows you what frequencies of radiation the molecules are absorbing.

Question 7

What can you use the spectrum produced by an infrared spectrometer for?

Answer 7

Identifying the functional groups in a molecule.

Question 8

What can you tell if you can use an IR spectrum to identify the functional groups in a molecule?

Answer 8

You can tell if a functional group has changed during a reaction.

Question 9

What can mass spectrometry help to identify?

Answer 9

Compounds.

Question 10

What can you get a mass spectra for?

Answer 10

Molecular samples.

Question 11

What is a mass spectrum produced by?

Answer 11

A mass spectrometer.

Question 12

What does a mass spectrometer produce?

Answer 12

A mass spectrum.

Question 13

What happens in mass spec?

Answer 13

The molecules in the sample are bombarded with electrons, which remove an electron from the molecule to form a molecular ion, M+ (g).

Question 14

What do you look at to find the relative molecular mass of a compound on a mass spectrum?

Answer 14

Molecular ion peak (the M peak).

Question 15

What can you find by looking at the molecular ion peak (the M peak)?

Answer 15

The relative molecular mass of a compound.

Question 16

How can you find out the relative molecular mass of a compound by looking at the molecular peak (the M peak)?

Answer 16

The mass/charge value of the molecular ion peak is the molecular mass.

Question 17

What is assumed when we say that the mass/charge value of the molecular ion peak is the molecular mass of the compound?

Answer 17

Assuming the ion has a 1+ charge, which it normally will have.

Question 18

What is the y-axis on an IR spectrum?

Answer 18

Transmittance %.

Question 19

What is the x-axis on an IR spectrum?

Answer 19

Wavenumber (cm^-1)

Question 20

What do the peaks on an IR spectrum show?

Answer 20

Where radiation is being absorbed.

Question 21

Which way are the peaks on IR spectra?

Answer 21

Upside-down (they point downwards).

Question 22

What is wavenumber?

Answer 22

The measure used for the frequency (it’s just 1/wavelength in cm.

Question 23

What is the y-axis on a mass spectrum?

Answer 23

The relative abundance of ions (often as a percentage).

Question 24

What is the x-axis on a mass spectrum?

Answer 24

Mass/charge (m/z)

Question 25

For most organic compounds, which peak is the M peak?

Answer 25

The one with the second highest mass/charge ratio.

Question 26

What is the smaller peak to the right of the M peak called?

Answer 26

The M+1 peak.

Question 27

What is the M+1 peak caused by?

Answer 27

The presence of the carbon isotope 13C.

Question 28

What can the molecular ion be broken down into?

Answer 28

Smaller fragments.

Question 29

What makes the molecular ions break up into fragments?

Answer 29

The bombarding electrons.

Question 30

What is shown on the mass spectrum?

Answer 30

The fragments that are ions - making a fragmentation pattern.

Question 31

What can you use fragmentation patterns to do?

Answer 31

Identify molecules and even their structure.

Question 32

Why aren’t free radicals shown on the mass spectrum?

Answer 32

They are ‘lost’.

Question 33

How do you work out the structural formula from the mass spectrum?

Answer 33

You have to work out what ion could have made each peak from its m/z value.

Question 34

What do you assume when working out the structural formula from a mass spectrum?

Answer 34

You assume that the m/z value of a peak matches the mass of the ion that made it.

Question 35

What is the molecular mass of the common fragment CH3+?

Answer 35

15

Question 36

What is the molecular mass of the common fragment C2H5+?

Answer 36

29

Question 37

What is the molecular mass of the common fragment CH3CH2CH2+?

Answer 37

43

Question 38

What is the molecular mass of the common fragment OH+?

Answer 38

17

Question 39

What common fragment makes up the molecular mass 15?

Answer 39

CH3+

Question 40

What common fragment makes up the molecular mass 29?

Answer 40

C2H5+

Question 41

What common fragment makes up the molecular mass 43?

Answer 41

CH3CH2CH2+

Question 42

What common fragment makes up the molecular mass 17?

Answer 42

OH+

Question 43

What can you combine IR spectroscopy and mass spectrometry to do?

Answer 43

Identify a compound.

Question 44

How do you combine IR spectroscopy and mass spectrometry to identify a compound?

Answer 44

1) Work out what functional groups are in a compound from its infrared spectrum.
2) Use the mass spectrum to work out the structure of the molecule.

Question 45

What is meant by molecular ion?

Answer 45

When an electron have been removed from the original molecule.

Question 46

Why do most organic compounds have a peak at about 3000 cm^-1 on IR spectra?

Answer 46

This is caused by the C-H groups.